In semiconductor processing, an integrated circuit is connected to its packaging by wires in a process known as wire bonding. During wire bonding it is necessary to clamp the wire to be bonded in order to position it precisely at the desired location. One known device for clamping wires is shown in FIG. 1, and includes a pair of jaws 102, 104 which are driven by a piezoelectric actuator 110. Applying an electric field across the actuator 110 causes the jaws 102, 104 to open. When the electric field is switched off, the arms close such that the wire is clamped between clamping plates 106, 108.
When using piezoelectric actuators it is necessary to apply a preload force to the piezoelectric element of the actuator before an electric field is applied. The magnitude of the preload force is important. If insufficient preload is applied, the piezoelectric element may break due to dynamic pull forces. If too much preload is applied, the actuator may not be able to successfully actuate.
In the wire clamp of FIG. 1, a preload force is applied by using a set screw 130 to apply a force to the piezo actuator 110, along its longitudinal axis (i.e, the axis running along the centre of the piezo actuator 110 and perpendicular to its ends 112, 114) via a preload block 120. The set screw 130 and preload block 120 push the piezo actuator 110 against the wire clamp main body 105.
A disadvantage of the prior art arrangement shown in FIG. 1 is that the screw 130 may become loose due to vibration or insufficient friction in the mating surfaces (i.e. between the screw 130 and preload block 120, and between the preload block 120 and piezo 110), resulting in a decrease of the preload force on the piezo actuator 110, and adversely affecting the performance of the wire clamp 100. In typical operation of a wire bonding apparatus employing the clamp 100, an external force will repeatedly be applied to the preloading screw 130 in the axial direction, which may cause the screw to loosen.
A further disadvantage of the preload mechanism of FIG. 1 is that a torque will be transmitted to the piezo stack 110 as the screw is tightened. This may damage the piezo stack 110 if not done carefully.
It would be desirable to provide a wire clamp which overcomes one or more of the above disadvantages, or which at least provides a useful alternative.